Mobile phones have become popular, in which smart phones having a touch screen applied thereto have become the mainstream. Smart phones use a lock screen to prevent “unintended execution of a function” that may occur when smartphones are in a pocket or bag. A user can unlock a locked screen by performing a specific input, for example, by flicking the screen or pressing an unlock button. After the locked screen is unlocked, the user can use a desired smart phone function. More particularly, the lock screen that is unlocked only when a password or specific pattern is input is used as a main means for protecting personal privacy.